With current and future market requirements for electrical components trending toward lighter weight plastic parts with improved electrical and mechanical properties, there is a substantial growth in the use of engineering plastics for electronic applications. At the present time, polyamides are the dominant engineering thermoplastic for electronic and other applications, especially when reinforced with a glass filler to increase their structural and impact strength and rigidity.
Polyamides are, in general, characterized as being relatively thermally stable upon long exposure to processing temperatures and shear. Upon exposure to flame, however, they burn quite readily, with the flammability being characterized by a dripping behavior of the burning resin. There is therefore a substantial and increasing demand for flame retardant polyamides and especially flame retardant glass-filled polyamides. Likewise, there is also demand for their polyester resin cousins, typically glass reinforced, with the choice of resin being dependent on several factors such as a balance between cost and mechanical property performance.
One of the major classes of flame retardants for thermoplastics and polyurethane foams is that of organic phosphorus compounds (typically phosphates and phosphonates). These may be non-halogenated or may include phosphorus-halogen compounds and blends of phosphorous compounds with halogenated flame retardants, typically brominated flame retardants.
In general organic phosphorus compounds provide fire retardant activity through a combination of condensed phase reactions, polymer carbonization promotion, and char formation. These processes obviously depend on the polymer in which such additive(s) reside. Therefore, specific phosphorus containing structures need to be designed for various polymers types
For example, U.S. Pat. No. 3,681,281 discloses a shaped structure comprising a polyester, at least 1 percent by weight of the polyester of a tertiary phosphine oxide, and from about 10 to about 50 percent by weight of the tertiary phosphine oxide of a synergist selected from the group consisting of triphenylmelamine, benzyl and dibenzyl. Among the tertiary phosphine oxides exemplified is xylylene bis-diphenylphosphine oxide.
In an article entitled “Phosphorus based additives for flame retardant polyester. 1. Low molecular weight additives”, Industrial & Engineering Chemistry Product Research and Development, (1982), 21(2), pages 328-31, Robert W Stackman evaluates various phosphorus-containing organic compounds, including xylylene bis-diphenylphosphine oxide, as flame retardants for poly(ethylene terephthalate) and poly(1,4-butylene terephthalate). The evaluation includes the effect of the additives on melt stability of preformed polymers as well as the effect upon flammability of films, as determined by a non-standard bottom burn, oxygen index method. The oxygen index values for these blends were a function of the phosphorus content of the blend. The efficiency of the phosphorus compounds as flame retardants changed as the nature of the phosphorus structure changed, with the order R3PO>R(R′O)2PO>(R′O)3PO. Polymeric additives were reported to be attractive additives, giving a combination of a high degree of flame retardancy combined with a minor degree of property degradation on blending, even at <20 wt % of the blend.
Other organic phosphorus compounds have also been suggested for use as flame retardants for polyamides. For example, Research Disclosure 168051 (published April 1978) entitled “Improved Nylon” describes a flame-resistant nylon fiber prepared by coating flakes of bis(4-aminocyclohexyl)methane-dodecanedioic acid copolymer with 8-10% p-xylylenebis(diphenylphosphine oxide) flame retardant and spinning a yarn using a screw melter equipped with a homogenizing, in-line mixer to supply the molten polymer to a unit for producing a 34-filament yarn. The molten polymer was heated to 300-10° and the holdup time was approximately 15 min. The yarn was drawn 2.3 times on a hot pipe to produce a 100-denier yarn.
In addition, U.S. Pat. No. 4,341,696 discloses a glass filled thermoplastic polyamide polymer rendered flame retardant by having combined therewith an effective amount of a tris-(3-hydroxyalkyl) phosphine oxide having the formula:
wherein R1 and R3 are any radical selected from the group consisting of hydrogen, phenyl and alkyl radicals of 1 to 4 carbon atoms and R2 is any radical selected from the group consisting of hydrogen, phenyl and alkyl radicals of 2 to 4 carbon atoms, provided that when R1 and R3 are hydrogen radicals, R2 is either an alkyl radical of 2 to 4 carbon atoms or a phenyl radical.
U.S. Pat. No. 7,332,534 discloses a flame retardant formulation for thermoplastic and thermoset polymers, including polyesters and polyamides, containing, as flame retardant component A, from 90 to 99.9% by weight of phosphinate salt of the formula:
and/or a diphosphinate salt of the formula
and/or polymers thereof, where R1, R2 are the same or different and are each C1-C6-alkyl, linear or branched, and/or aryl; R3 is C1-C10-alkylene, linear or branched, C6-C10-arylene, -alkylarylene or -arylalkylene; M is Mg, Ca, Al, Sb, Sn, Ge, Ti, Zn, Fe, Zr, Ce, Bi, Sr, Mn, Li, Na and/or K; m is from 1 to 4; n is from 1 to 4; x is from 1 to 4 and, as component B, from 0 to 50% by weight of a nitrogen-containing synergist or of a phosphorus/nitrogen flame retardant and, as component C, from 0.1 to 10% by weight of a liquid component.
U.S. Pat. No. 7,411,013 discloses a flame-retardant resin composition comprising a base resin (A), such as a polyester, polyamide or styrenic resin, an organic phosphorus compound (B) and a flame-retardant auxiliary (C), wherein the organic phosphorus compound (B) has a unit represented by the following formula:
wherein Ar represents an aromatic hydrocarbon ring or a nitrogen-containing aromatic heterocycle; X1 represents an oxygen atom or a sulfur atom; Y1 and Y2 are the same or different and each represents a hydrocarbon group, an alkoxy group, an aryloxy group, or an aralkyloxy group; Z1 represents an alkylene group, or a nitrogen-containing bivalent group corresponding to an alkylamine; Y1 and Y2 may bind to each other, and Y1 and Y2 together with the adjacent phosphorus atom may form a ring; “a” denotes 0 or 1; and “b” denotes an integer of 1 to 6.
According to the present invention, it has now been found that certain benzyl-substituted phosphorus oxide compounds, especially when combined with specific synergists, are highly effective flame retardants for thermoplastic resins, including polyamides, especially glass filled polyamides.